The Structure and Function of Nucleic Acids

1 The Structure and Functionof Nucleic AcidsRevised Bryce* and D. Pacini *Department of Biological Sciences, Napier University, Edinburgh, and Bolton School Boy s Division, BoltonThe Biochemistry Across the School Curriculum Group (BASC) was setup by the Biochemical Society in 1985. Its membership includes edu-cation professionals as well as Society members with an interest inschool science education. Its first task has been to produce this series ofbooklets, designed to help teachers of syllabuses which have a high bio-chemical topics covered by this series include: Essential Chemistry forBiochemistry; Enzymes and their Role in Biotechnology; Metabolism;Immunology; Photosynthesis;Recombinant DNA Technology; BiologicalMembranes.

2 And The Biochemical Basis of information on the work of BASC and these booklets is availablefrom the Education Officer at the Biochemical Society, 59 PortlandPlace, London W1N on the content of this booklet will be welcomed by the SeriesEditor Mrs D. Gull at the above 0 90449 834 4 The Biochemical Society 1998 First edition published 1991 All BASC material is copyright by the Biochemical Society. Extractsmay be photocopied for classroom work, but complete reproduction ofthe entire text or incorporation of any of the material with other doc-uments or coursework requires approval by the Biochemical by Holbrooks Printers Ltd, Portsmouth, Acids and genetic information transfer.

3 1 Learning and Structure of Nucleic Acids ..5 Learning ..7 Inorganic Nucleic Acids from their building di- and triphosphates ..9 Nucleic Acids ..10 DNA isolation, base composition and Structure ..12 RNA isolation, base composition and objectives ..21 Introduction ..21 The Meselson and Stahl experiment ..23 Mechanism of genetic objectives ..29 Introduction ..29 How many bases code for an amino acid? ..31 Evidence for a triplet code ..32 Deciphering the codons ..34iv5 Transcription ..37 Learning objectives ..37 Introduction ..37 Mechanism of transcription .. objectives.

4 41 Introduction ..41 Activation of ..43 Elongation ..44 Termination ..45 Polysomes ..467 Assessment: past examination questions and ..47 Answers ..568 Laboratory practicals ..59 Extraction and characterization of DNA from of RNA ..61 Further practicals ..629 Source materials ..63 Textbooks ..63 Articles ..63 Teaching index ..65vForewordA study of the Structure and Function of Nucleic Acids is needed to beable to understand how information controlling the characteristics of anorganism is stored in the form of genes in a cell and how these genes aretransmitted to future generations of offspring.

5 The rapid developmentsin the area of genetic engineering and recombinant DNA technology(which are covered in Booklet 7 of the BASC series) have only beenpossible as a result of detailed understanding of the Structure of DNAand RNA. It is therefore not surprising to see Nucleic Acids included inthe compulsory core subject matter of all of the linear and modular A-level Biology syllabuses. The major purpose of these guidance notes is toprovide an account of the role of DNA and RNA in these processes in atleast sufficient detail for A-level study. Additional information moresuited for general interest, Oxbridge preparation or first-year under-graduate study is placed in shaded boxes of order to maximize the benefit of this booklet for A-level studentsand their teachers, a set of learning objectives have been included at thestart of chapters 1 6.

6 These objectives are based upon the relevantknowledge and understanding statements of current A-level Biologysyllabuses. Chapter 7 consists of recent A-level examination questionsand answers and this should help students get a feel for the level of detailrequired. We are grateful to the various Examinations Boards, especiallythe Northern Examinations and Assessment Board (NEAB), for theirpermission to include this Acids and geneticinformation transferLearning objectivesEach student should, without reference to his or her notes, be able to: state that the genotype describes the genetic composition of an organism in terms of specific genes, alleles that it contains; state that the phenotype is the collection of visible characteristics which identify an individual organism.

7 State that the phenotype of an individual depends not only on its genotype but also on the environment in which it lives; and state that DNA, which is found in the nucleus of a cell, contains and is responsible for the transfer of genetic information, the , when we talk about genes and heredity we perhaps thinkimmediately of how children resemble their parents or, depending onour background knowledge, we may even think about Mendel and theheight and colour of his pea plants. This level of genetics is known asphenotypic expression, where the phenotypeof an individual is the col-lection of visible and recognizable characteristics by which that indi-vidual is phenotype is influenced by the environment as well as thegenotype; but this aspect is more appropriate to classical genetics and willnot concern us at the present time.

8 In this section we are more concernedwith the collection of the inherited factors which determine these traits,1and this is a cell s genotypic expression. The genotypeis the genetic com-position of an organism in terms of the forms of specific genes, alleles,that it contains, and its study is referred to as molecular very fact that a child can resemble one or both parents meansthat there was some way in which the genetic information was passedfrom the parent to the child. What then is the nature of the geneticmaterial? The answer to this question came from the experimentalobservations shown in Table to heredity than DNAIn similar types of nuclear transplantation experiments, scientists have recentlyrevealed that the environment of the nucleus ( the cytoplasm in which it findsitself) can alter the functioning of key genes, including those which determinebody have been referred to as epigeneticchanges, these beingchanges which affect the way DNA works rather than sequence could have implications for in vitrofertilization in Scientist(19 April 1997)

9 Single-cell organisms take a siestaIn 1997 a short report appeared which indicated that a study on the patterns ofDNA synthesis in single-cell organisms revealed that cells synthesized DNA fromsunrise to about noon, then shut down synthesis for between three and sixhours, then restarted before explanation given for this phenomenon isthat this represents an evolutionary throwback to the Precambrian times whenthere was no ozone layer and hence the sun s ultraviolet light at its peak couldhave damaged the may have a future significance if the ozone layerdoes continue to Scientist(4 January 1997)Such studies represent the most conclusive evidence that DNA isresponsible for the transfer of genetic information, at least in micro-organisms and viruses.

10 The evidence that this is also true in higherorganisms (plants and animals), although somewhat less conclusive,leaves no doubt that this is the case. Since some virus particles have beenfound that have no DNA, but which have RNA instead, it is morecorrect to say that Nucleic Acids are the genetic information carriers .The exact roles of DNA and RNA in the complex process of thetransfer of genetic information are the subjects of subsequent sections ofthis Acids : Structure and functionNucleic Acids and genetic information transfer3 Table 1. Steps in the discovery of DNA as the genetic materialExperimenter(s) and datesDetails of procedureConclusionLate 19th century biologistsRemoval of the nucleus of the cell produced death, Something in the nucleus is responsible for a but removal of the same volume of cytoplasm did s 19th century biologistsRemoval of a cell nucleus followed by transplantation Something in the nucleus is responsible for of the nucleus from a different cell type changed the the phenotype of a and Function of the donor (1929)